Gear oils and transmission fluids have been required recently to provide effective lubrication even in systems which are exposed to longer drain intervals and to higher temperatures. When the lubricant is exposed to these high temperatures, oxidation of the base oil occurs, usually at the points of unsaturation of the oils used in the lubricant. This oxidation leads to unacceptable viscosity change (increase) and reduced effectiveness of the lubricants.
Lubricant formulators have sought to provide lubricants which have improved oxidation stability. Polyalphaolefins have been used as an alternative to mineral oils. The polyaphaolefins have low levels unsaturation and have good high temperature performance properties. However, polyalphaolefins are more expensive than mineral oils. Also, the polyalphaolefins generally have better low temperature performance that mineral oils.
Mineral oils have been developed which provide improved oxidation properties. These mineral oils are generally referred to as Group II and Group III basestocks. These basestocks were developed for passenger car lubricants.
These basestocks cost less than polyalphaolefins. The Group II basestocks are generally worse in oxidation and low temperature performance than Group III basestocks. Group II and III basestocks have a high level of saturation. However, Applicants have discovered that oxidation and low temperature performance are affected by the type of saturated components in the oil. Group II stocks generally have a higher proportion of cyclic saturated components than aliphatic saturated components.
The type of environment affects the oxidation of basestocks. In the passenger car motor oil area, the form of oxidation is caused by the increased acidic blow by-products from combustion of fuel in the engines, either diesel or gasoline fuel. These acidic combustion products act together with oxygen to enhance oxidation of the oils. In the gear and transmission area, the oxidation is different than those in passenger car motor oils. There are no acidic blow by-products to enhance oxidation of the oils.
It is desirable to have a Group III based transmission or gear lubricant which provides good oxidation performance.